1. Field
The present disclosure relates to a tire with a radial carcass reinforcement, and more particularly a tire intended for fitting to vehicles carrying heavy loads and travelling at sustained speeds, such as lorries, tractors, trailers or buses.
2. Description of Related Art
In the tires of heavy goods vehicles, the carcass reinforcement is generally fixed on either side in the area of the bead and is surmounted radially by a crown reinforcement composed of at least two layers, superimposed and formed of parallel threads or cords in each layer, with each layer crossing the next at angles in the range from 10° to 45° to the circumferential direction. Said working layers forming the working reinforcement may be further covered by at least one layer, called the protective layer, formed by reinforcing elements which are advantageously metallic and extensible and are called elastic. It may also comprise a layer of metal threads or cords having low extensibility, forming an angle in the range from 45° to 90° to the circumferential direction, this layer, called the triangulation layer, being radially located between the carcass reinforcement and the first crown ply, formed by parallel threads or cables lying at angles not exceeding 45° in absolute terms. The triangulation ply forms a triangulated reinforcement with at least said working ply, this reinforcement having low deformation under the various stresses which it undergoes, the triangulation ply essentially serving to absorb the transverse compressive forces acting on all the reinforcing elements in the crown area of the tire.
Cords are called inextensible if said cords have a relative elongation of not more than 0.2% under a tensile load equal to 10% of the breaking load.
Cords are called elastic if said cords have a relative elongation of at least 3% under a tensile load equal to the breaking load, with a maximum tangent modulus of less than 150 GPa.
Circumferential reinforcing elements are reinforcing elements which form angles to the circumferential direction in the range +2.5°, −2.5° around 0°.
The circumferential direction of the tire, or the longitudinal direction, is the direction which corresponds to the periphery of the tire and which is defined by the rolling direction of the tire.
The transverse or axial direction of the tire is parallel to the axis of rotation of the tire.
The radial direction is a direction cutting the axis of rotation of the tire and perpendicular thereto.
The axis of rotation of the tire is the axis around which it revolves in normal use.
A radial or meridian plane is a plane containing the axis of rotation of the tire.
The circumferential mid-plane, or equatorial plane, is a plane which is perpendicular to the axis of rotation of the tire and divides the tire into two halves.
The “modulus of elasticity” of a rubber mixture is considered to be a secant modulus of extension at 10% deformation and at ambient temperature.
Where rubber compositions are concerned, measurements of the modulus are performed under tension according to the AFNOR-NFT-46002 standard of September 1988: the nominal secant modulus (or apparent stress, in MPa) is measured in a second elongation (i.e. after an adaptation cycle) at 10% elongation (in normal conditions of temperature and humidity according to the AFNOR-NFT-40101 standard of December 1979).
Some present-day tires, called “road tires”, are intended to roll at high speed over increasingly long distances, because of the improvement of the road network and the growth of the worldwide road network. Unquestionably, the set of conditions in which a tire of this type is required to roll enables the mileage covered to be increased; however, the endurance of the tire, and particularly that of the crown reinforcement, is adversely affected.
This is because stresses are present at the position of the crown reinforcement; more particularly, there are shear stresses between the crown layers, combined with a non-negligible rise in the operating temperature at the ends of the axially shortest crown layer, resulting in the appearance and propagation of cracks in the rubber at said ends.
In order to improve the endurance of the crown reinforcement of the type of tire in question, solutions have already been proposed regarding the structure and quality of the layers and/or profiled elements of rubber mixtures which are placed between and/or around the ends of the plies, and more particularly the ends of the axially shortest ply.
There is, notably, a known way of introducing a layer of rubber mixture between the ends of the working layers to create decoupling between said ends in order to limit the shear stresses. However, these decoupling layers must have a very high degree of cohesion. These layers of rubber mixtures are described, for example, in patent application WO2004/076204.
In order to improve the resistance to degradation of rubber mixtures located in the vicinity of the edges of the crown reinforcement, the patent FR 1 389 428 recommends the use, in combination with a low-hysteresis tread, of a rubber profiled element covering at least the sides and marginal edges of the crown reinforcement and composed of a rubber mixture with low hysteresis.
In order to avoid separations between the crown reinforcement plies, patent FR 2 222 232 teaches the embedding of the reinforcement ends in a rubber pad whose Shore A hardness is different from that of the tread surmounting said reinforcement, and greater than the Shore A hardness of the profiled element of rubber mixture placed between the edges of the crown reinforcement plies and the carcass reinforcement.
Tires made in this way can indeed improve performance, notably in terms of endurance.
There is also a known way of making tires with a very wide tread, or introducing a layer of circumferential reinforcing elements to impart greater load-bearing capacity to tires of a given size. Patent application WO 99/24269 describes, for example, the presence of a layer of circumferential reinforcing elements of this type.
The layer of circumferential reinforcing elements is usually formed by at least one metallic cord wound to form a turn whose angle of lay with respect to the circumferential direction is less than 2.5°.